to feel that everyone is trying to undermind me

[dubbada]

DP gives in to ds tantrums, ignores bad behaviour
GPs give sweets depite being told that hes been naugty
DS switches from hellboy attitude to angelic cherub in seconds, generally attacking his poor sis.

Urggh feel like im not in control of anything and its to bl**dy early to open the wine!

Aw, feel for you but there is no other way than to address it. If GPs offer sweets in those circs, say that you will keep them for later as DS has been naughty. (I have to do this with my sister!) and have it out with DP, explain that you both have to sing from same hymn sheet or it won't work. Good luck.

What Mopswerver said.

My mil does this. I tend to jump in and take the sweets and say ' as I said, when he can behave he can have the sweets' and then stare them down.
The same for when they undermine me if I tell him off.

Its bloody annoying

I truly am doing the best i can, but am accused of either being to strict or not dealing with it and so starts the wonderful circle of self doubt defensiveness

DP needs to be on the same page as you, otherwise it will get worse. If he works FT and you are a SAHM then he might not understand how damaging his behaviour can be.

PIL you do have a say over.

Generally I try repetition and routine (you probably do this already). it helps me explain to DD (who is also being a bit naughty of late) what is happening and what consequences will happen if she misbehaves. I've been quite strict with her and have had to get DH to go along with me, but it is working and we are finally getting somewhere.

Good luck OP, it's amazing how the children we love so much, can push us so much!